Semiconductor device

ABSTRACT

The semiconductor device according to one embodiment includes a power transistor and a sense transistor connected in parallel with each other, a first operational amplifier having a non-inverting input terminal connected to an emitter of the sense transistor and an inverting input terminal connected to an emitter of the power transistor, a resistor element having one end connected to the emitter of the sense transistor and another end connected to a first node, and an adjustment transistor placed between the first node and a low-voltage power supply. The first operational amplifier adjusts a current flowing through the adjustment transistor so that an emitter voltage of the power transistor and an emitter voltage of the sense transistor are substantially the same.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2014-190819, filed on Sep. 19, 2014, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

The present invention relates to a semiconductor device and, forexample, relates to a semiconductor device including a power transistor.

A power transistor is widely used today as an element that passes acurrent through a load requiring a large current. As the powertransistor, an element using a bipolar transistor or an element usingMOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is used, forexample. Particularly, IGBT (Insulated Gate Bipolar Transistor) iscommonly used.

A device in which a power transistor is used is provided with a circuitthat detects an overcurrent in order to prevent breakdown of the devicedue to short-circuit of a load. For example, by placing a sensetransistor smaller than a power transistor in parallel with the powertransistor and monitoring a current flowing through the emitter of thesense transistor, it is possible to monitor a current flowing throughthe power transistor. For example, by placing a shunt resistor on theemitter side of the sense transistor and monitoring a voltage betweenterminals of the shunt resistor, it is possible to monitor a currentflowing through the emitter of the sense transistor.

However, if the shunt resistor is placed, the emitter voltage of thesense transistor differs from the emitter voltage of the powertransistor under the effect of the voltage occurring at the shuntresistor. Therefore, a current detection circuit that is formed usingthe sense transistor and the shunt resistor has a problem that thedetection accuracy is low.

In the technique disclosed in Japanese Unexamined Patent ApplicationPublication No. H11-299218, an operational amplifier that virtuallyshort-circuits the emitter of the power transistor and the emitter ofthe sense transistor is placed in order to solve the above problem. Byplacing such an operational amplifier, it is possible to substantiallyequalize the emitter voltage of the power transistor and the emittervoltage of the sense transistor and thereby improve the detectionaccuracy of the current detection circuit.

SUMMARY

A current flowing through the sense transistor is smaller than a currentflowing through the power transistor; however, because a large currentflows through the power transistor, a certain level of current flowsthrough the sense transistor as well. For example, when the ratio (senseratio) between a current flowing through the emitter of the powertransistor and a current flowing through the emitter of the sensetransistor is 1000:1, if a current of 400 A flows through the emitter ofthe power transistor, a current of 400 mA flows through the emitter ofthe sense transistor.

In the technique disclosed in Japanese Unexamined Patent ApplicationPublication No. H11-299218, by virtually short-circuiting the emitter ofthe power transistor and the emitter of the sense transistor using theoperational amplifier, the emitter voltage of the power transistor andthe emitter voltage of the sense transistor are substantially equalized.The output terminal of the operational amplifier is electricallyconnected to the emitter of the sense transistor, and a current flowingthrough the emitter of the sense transistor is taken using theoperational amplifier. Therefore, there is a problem that, when acurrent flowing through the sense transistor increases, the operationalamplifier generates heat, and the current detection circuit thatincludes the operational amplifier also generates heat.

The other problems and novel features of the present invention willbecome apparent from the description of the specification and theaccompanying drawings.

A semiconductor device according to one embodiment includes a powertransistor and a sense transistor connected in parallel with each other,a first operational amplifier having a first input terminal connected toan emitter of the sense transistor and a second input terminal connectedto an emitter of the power transistor, a resistor element having one endconnected to the emitter of the sense transistor and another endconnected to a first node, and an adjustment transistor placed betweenthe first node and a low-voltage power supply. The first operationalamplifier adjusts a current flowing through the adjustment transistor sothat an emitter voltage of the power transistor and an emitter voltageof the sense transistor are substantially the same.

According to the above embodiment, it is possible to provide asemiconductor device that can suppress the heating of a currentdetection circuit even when a large current flows through a powertransistor.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, advantages and features will be moreapparent from the following description of certain embodiments taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a circuit diagram showing one example of a semiconductordevice according to a first embodiment.

FIG. 2 is a circuit diagram showing another example of a semiconductordevice according to the first embodiment.

FIG. 3 is a circuit diagram showing another configuration example of apower transistor circuit.

FIG. 4 is a circuit diagram showing one example of a semiconductordevice according to a second embodiment.

FIG. 5 is a circuit diagram showing one example of a semiconductordevice according to a third embodiment.

FIG. 6 is a diagram illustrating an operating state of the semiconductordevice according to the third embodiment.

FIG. 7 is a circuit diagram illustrating a comparative example.

FIG. 8 is a graph showing a relationship between a current I_(CE)flowing through a main emitter of the power transistor and a voltageV_(RS) in a shunt resistor shown in FIG. 7.

FIG. 9 is a circuit diagram illustrating a semiconductor deviceaccording to a comparative example.

DETAILED DESCRIPTION

<First Embodiment>

A first embodiment is described hereinafter with reference to thedrawings. FIG. 1 is a circuit diagram showing one example of asemiconductor device according to the first embodiment. As shown in FIG.1, a semiconductor device 1 according to this embodiment includes apower transistor circuit 11, a current detection circuit 12, a shuntresistor Rs, and an adjustment transistor Tr3.

The power transistor circuit 11 includes a power transistor Tr1 and asense transistor Tr2. The sense transistor Tr2 is an element that isplaced for detecting a current flowing through the emitter of the powertransistor Tr1. A current that flows through the emitter SE of the sensetransistor Tr2 is smaller than a current that flows through the emitterE of the power transistor Tr1. As one example, the ratio (sense ratio)between a current that flows through the emitter E of the powertransistor Tr1 and a current that flows through the emitter SE of thesense transistor Tr2 may be approximately 1000:1. The gate of the sensetransistor Tr2 is connected to the gate of the power transistor Tr1, andthe collector of the sense transistor Tr2 is connected to the collectorof the power transistor Tr1. In other words, the sense transistor Tr2 isconnected in parallel with the power transistor Tr1.

A load (not shown) is connected to the emitter E of the power transistorTr1. An emitter current I_(CE) flows through the emitter E of the powertransistor Tr1. Further, the emitter KE of the power transistor Tr1(which is a Kelvin emitter in this case) is connected to the invertinginput terminal (second input terminal) of an operational amplifier AMP1(first operational amplifier) included in the current detection circuit12. The emitter SE of the sense transistor Tr2 is connected to thenon-inverting input terminal (first input terminal) of the operationalamplifier AMP1. One end of the shunt resistor Rs (resistor element) isconnected to the emitter SE of the sense transistor Tr2, and the otherend of the same is connected to a node N1.

The adjustment transistor Tr3 is placed between the node N1 and alow-voltage power supply Vss. Specifically, the collector of theadjustment transistor Tr3 is connected to the node N1, the emitter ofthe adjustment transistor Tr3 is connected to the power supply Vss, andthe output CG of the operational amplifier AMP1 is supplied to the baseof the adjustment transistor Tr3. Note that the adjustment transistorTr3 may be formed using MOSFET. When using MOSFET, the drain of theadjustment transistor Tr3 is connected to the node N1, the source of theadjustment transistor Tr3 is connected to the power supply Vss, and theoutput CG of the operational amplifier AMP1 is supplied to the gate ofthe adjustment transistor Tr3. Hereinafter, the case where theadjustment transistor Tr3 is formed using MOSFET is described as oneexample.

The operational amplifier AMP1 is placed in the current detectioncircuit 12. The current detection circuit 12 is formed on asemiconductor chip (IC) which is different from a semiconductor chip(IC) that forms the power transistor circuit 11. Further, the adjustmenttransistor Tr3 is placed outside the current detection circuit 12 (whichis outside the semiconductor chip including the current detectioncircuit 12).

The voltage of the node N1 corresponds to the current I_(CE) flowingthrough the power transistor Tr1, and it is possible to monitor thecurrent I_(CE) flowing through the power transistor Tr1 by monitoringthe voltage of the node N1.

In this embodiment, in order to keep the voltage of the emitter SE(which is also referred to hereinafter as a sense emitter SE) of thesense transistor Tr2 to be substantially the same as the voltage of themain emitter E of the power transistor Tr1, feedback control of thevoltage of one end of the shunt resistor Rs (which is the voltage of thesense emitter SE) is performed using the operational amplifier AMP1.Specifically, the operational amplifier AMP1 adjusts a current flowingthrough the adjustment transistor Tr3 so that the voltage of the mainemitter E and the voltage of the sense emitter SE are substantially thesame. In this embodiment, the Kelvin emitter KE is placed to acquire thevoltage of the main emitter E. The Kelvin emitter KE is a line that isdrawn from the near point to an element that forms the power transistorTr1. The voltage of the main emitter E is the same as the voltage of theKelvin emitter KE, and “the voltage of the main emitter E” and “thevoltage of the Kelvin emitter KE” are hereinafter used as the samemeaning.

When the voltage of the sense emitter SE is higher than the voltage ofthe Kelvin emitter KE, the output voltage CG of the operationalamplifier AMP1 becomes higher, and the gate voltage of the adjustmenttransistor Tr3 increases. A current flowing through the adjustmenttransistor Tr3 thereby increases, and the voltage of the sense emitterSE decreases. On the contrary, when the voltage of the Kelvin emitter KEis higher than the voltage of the sense emitter SE, the output voltageCG of the operational amplifier AMP1 becomes lower, and the gate voltageof the adjustment transistor Tr3 decreases. A current flowing throughthe adjustment transistor Tr3 thereby decreases, and the voltage of thesense emitter SE increases. Then, it balances out when the input voltageof the non-inverting input terminal and the input voltage of theinverting input terminal of the operational amplifier AMP1 (which arethe voltage of the sense emitter SE and the voltage of the Kelvinemitter KE) become the same.

By placing the operational amplifier AMP1 in this manner, the voltage ofthe sense emitter SE can be kept substantially the same as the voltageof the Kelvin emitter KE, and it is thereby possible to align the biasconditions of the power transistor Tr1 and the sense transistor Tr2 (theboth transistors are in the same semiconductor substrate). Accordingly,it is possible to keep the sense ratio of the power transistor Tr1 andthe sense transistor Tr2 (the ratio of a current flowing through theemitter E of the power transistor Tr1 and a current flowing through thesense emitter SE of the sense transistor Tr2) constant regardless of thetemperature, gate voltage and collector voltage. As a result, a currentproportional to a current flowing through the emitter E of the powertransistor Tr1 flows through the sense emitter SE. Thus, due to avoltage drop by the shunt resistor Rs, a voltage proportional to acurrent flowing through the emitter E of the power transistor Tr1 occursat the node N1. When a current flowing through the emitter E of thepower transistor Tr1 is I_(CE), a current flowing through the senseemitter SE is I_(CSE), a sense ratio is γ, and a voltage of the node N1is V_(SC), V_(SC)=−Rs·I_(CSE)=−Rs·γ·I_(CE). Thus, as a current flowingthrough the emitter E of the power transistor Tr1 becomes larger, avoltage of the node N1 decreases.

At this time, the configuration is constructed so that the low-voltagepower supply Vss of the operational amplifier AMP1 and the low-voltagepower supply Vss connected with the source of the adjustment transistorTr3 are lower than the voltage of the Kelvin emitter KE of the powertransistor Tr1.

Particularly, in this embodiment, it is preferred that the low-voltagepower supply Vss of the operational amplifier AMP1 is a negativevoltage. Specifically, as shown in a semiconductor device 1′ of FIG. 2,the inverting input terminal of the operational amplifier AMP1 may beconnected to a ground potential (GND), and the source of the adjustmenttransistor Tr3 may be connected to a negative potential. In this case,because the non-inverting input terminal of the operational amplifierAMP1 acts as a virtual ground, the voltage of the sense emitter SEconverges to the same potential as the voltage of the Kelvin emitter KE.

To be specific, in the configuration example shown in FIG. 2, thevoltage of the sense emitter SE needs to be kept the same as the voltageof the Kelvin emitter KE with a current flowing through the shuntresistor Rs. Because the main emitter E is generally connected to theGND of the gate driver circuit, it is necessary to supply a lowernegative voltage than that of the sense emitter SE to the other end(node N1) of the shunt resistor Rs. Therefore, the low-voltage powersupply Vss of the operational amplifier AMP1 and the source of theadjustment transistor Tr3 are connected to the negative power supply.

For example, as shown in FIG. 2, a negative voltage generation circuit15 may be placed in a current detection circuit 12′ (which is in asemiconductor chip that forms the current detection circuit 12′) as thenegative power supply. The negative voltage generation circuit 15 may beformed using a charge pump circuit, for example. In this manner, byplacing the negative voltage generation circuit 15 in the currentdetection circuit 12′, it is possible to reduce the number of externalparts and thereby reduce the manufacturing cost.

FIG. 3 is a circuit diagram showing another configuration example of thepower transistor circuit 11. The power transistor circuit 11 shown inFIG. 1 has a configuration in which the power transistor Tr1 and thesense transistor Tr2 are formed using different transistors. However, inthis embodiment, some emitters of a plurality of transistor elementsthat form a power transistor Tr11 may be used as the sense emitter SE asshown in a power transistor circuit 11_1 in FIG. 3. Specifically, thepower transistor Tr11 has a configuration in which a plurality oftransistor elements are connected in parallel, and the emitters of theplurality of transistor elements may be divided into the main emitter Eand the sense emitter SE. In this case, the number of emitters that formthe sense emitter SE is smaller than the number of emitters that formthe main emitter E. The ratio of the number of emitters that form themain emitter E and the number of emitters that form the sense emitter SEcorresponds to the sense ratio.

Further, as shown in a power transistor circuit 11_2 in FIG. 3, thepower transistor circuit may be formed using MOSFET in this embodiment.Specifically, the power transistor circuit 11_2 includes a power MOSFET(Tr12) and a sense MOSFET (Tr13). The gate of the sense MOSFET (Tr13) isconnected to the gate of the power MOSFET (Tr12), and the drain of thesense MOSFET (Tr13) is connected to the drain of the power MOSFET(Tr12). In other words, the sense MOSFET (Tr13) is connected in parallelwith the power MOSFET (Tr12).

The power transistor circuit 11_2 has a configuration in which the powerMOSFET (Tr12) and a sense MOSFET (Tr13) are formed using differentMOSFET as an example. However, in this embodiment, some sources of aplurality of transistor elements that form a power MOSFET (Tr14) may beused as a sense source SS as shown in a power transistor circuit 11_3 inFIG. 3. Specifically, the power MOSFET (Tr14) has a configuration inwhich a plurality of transistor elements are connected in parallel, andthe sources of the plurality of transistor elements may be divided intothe main source S and the sense source SS. In this case, the number ofemitters that form the sense source SS is smaller than the number ofemitters that form the main source S. The ratio of the number of sourcesthat form the main source S and the number of sources that form thesense source SS corresponds to the sense ratio.

As described above, MOSFET can be used as the transistor in thisembodiment. In this specification, each terminal of the transistor isreferred to as “base or gate”, “collector or drain” or “emitter orsource” in some cases.

In the technique disclosed in Japanese Unexamined Patent ApplicationPublication No. H11-299218, in order to improve the detection accuracyof the current detection circuit that detects a current flowing throughthe power transistor, the emitter of the power transistor and theemitter of the sense transistor are virtually short-circuited using theoperational amplifier, so that the emitter voltage of the powertransistor and the emitter voltage of the sense transistor aresubstantially equalized. However, in the technique disclosed in JapaneseUnexamined Patent Application Publication No. H11-299218 the outputterminal of the operational amplifier is electrically connected to theemitter of the sense transistor, and a current flowing through theemitter of the sense transistor is taken using the operationalamplifier. Therefore, there is a problem that, when a current flowingthrough the sense transistor increases, the operational amplifiergenerates heat, and the current detection circuit that includes theoperational amplifier also generates heat.

In view of the above, in this embodiment, the adjustment transistor Tr3is placed between the node N1 and the low-voltage power supply Vss asshown in FIGS. 1 and 2. Then, the output of the operational amplifierAMP1 is supplied to the gate of the adjustment transistor Tr3, and acurrent flowing through the adjustment transistor Tr3 is adjusted sothat the voltage of the Kelvin emitter KE and the voltage of the senseemitter SE are substantially the same. Therefore, even when a currentflowing through the sense transistor Tr2 becomes large, it is possibleto prevent heating of the operational amplifier AMP1. Specifically, whena current flowing through the sense transistor Tr2 becomes large, acurrent flowing through the adjustment transistor Tr3 also becomeslarge; however, because this does not affect the operational amplifierAMP1, it is possible to prevent the operational amplifier AMP1 fromgenerating heat.

Further, in this embodiment, the adjustment transistor Tr3 is placedoutside the current detection circuit 12. Specifically, the adjustmenttransistor Tr3 is placed on a semiconductor chip (IC) different from thesemiconductor chip (IC) including the operational amplifier AMP1.Therefore, even when a current flowing through the adjustment transistorTr3 becomes large (that is, when a current flowing through the sensetransistor Tr2 becomes large), and the heating value of the adjustmenttransistor Tr3 increases, it is possible to prevent the heating of theadjustment transistor Tr3 from affecting the current detection circuit12. Further, in this case, it is only necessary to change the size ofthe adjustment transistor Tr3 in accordance with the size of the powertransistor Tr1 to be used, and therefore the semiconductor chip (IC) canbe easily generalized.

FIG. 7 is a circuit diagram illustrating a comparative example, and itshows a circuit for inspecting the relationship between a current I_(CE)flowing through the main emitter ME of a power transistor Tr21 and acurrent I_(CE2) flowing through the sense emitter SE. As shown in FIG.7, a gate control signal is supplied from a control signal generationcircuit 91 to the gate of the power transistor Tr21. A current issupplied from a current source 94 to the collector of the powertransistor Tr21. A shunt resistor R_(S31) is connected to the senseemitter SE. A voltage V_(RS) between the both terminals of the shuntresistor R_(S31) is measured using a voltmeter 92. The voltage V_(RS)corresponds to the current I_(CE2) flowing through the sense emitter SE.The current I_(CE) flowing through the main emitter ME is measured usingan ammeter 93.

FIG. 8 is a graph showing the relationship between the current I_(CE)flowing through the main emitter ME of the power transistor Tr21 and thevoltage V_(RS) between the both terminals of the shunt resistor R_(S31)shown in FIG. 7. As shown in FIG. 8, the current I_(CE) flowing throughthe main emitter ME and the voltage V_(RS) in the shunt resistor R_(S31)are proportional. However, the ratio (sense ratio) of the current I_(CE)flowing through the main emitter ME and the shunt resistor voltageV_(RS) depends on temperature (junction temperature). Specifically, whenthe junction temperature of the power transistor Tr21 becomes higher, athreshold voltage of the power transistor Tr21 becomes lower, and theshunt resistor voltage V_(RS) with respect to the current I_(CE) varies(the voltage of the sense emitter SE becomes higher than the voltage ofthe main emitter ME). In this manner, the current detection circuitformed using the sense transistor and the shunt resistor has a problemthat the detection accuracy is low because the shunt resistor voltageV_(RS) varies with a change in the junction temperature of the powertransistor Tr21.

FIG. 9 is a circuit diagram illustrating a semiconductor deviceaccording to a comparative example, and it shows a semiconductor devicethat can solve the above problem. A semiconductor device 101 shown inFIG. 9 includes a power transistor circuit 111, a current detectioncircuit 112, and a shunt resistor Rs.

The power transistor circuit 111 includes a power transistor Tr31 and asense transistor Tr32. The gate of the sense transistor Tr32 isconnected to the gate of the power transistor Tr31, and the collector ofthe sense transistor Tr32 is connected to the collector of the powertransistor Tr31. In other words, the sense transistor Tr32 is connectedin parallel with the power transistor Tr31.

A load (not shown) is connected to the emitter E of the power transistorTr31. An emitter current I_(CE) flows through the emitter E of the powertransistor Tr31. Further, the Kelvin emitter KE of the power transistorTr31 is connected to the non-inverting input terminal of an operationalamplifier AMP10 included in a current detection circuit 112. The emitterSE of the sense transistor Tr32 is connected to the inverting inputterminal of the operational amplifier AMP10. One end of the shuntresistor Rs is connected to the emitter SE of the sense transistor Tr32,and the other end of the shunt resistor Rs is connected to the outputterminal of the operational amplifier AMP10.

In this manner, the semiconductor device 101 according to thecomparative example shown in FIG. 9 includes the operational amplifierAMP10 that virtually short-circuits the Kelvin emitter KE and the senseemitter SE in order to prevent the voltage of the sense emitter SE frombeing different from the voltage of the Kelvin emitter KE. Specifically,by placing the operational amplifier AMP10, the voltage of the Kelvinemitter KE and the voltage of the sense emitter SE can be substantiallythe same, and it is thereby possible to improve the detection accuracyof the current detection circuit.

However, in the semiconductor device 101 shown in FIG. 9, the outputterminal of the operational amplifier AMP10 is electrically connected tothe sense emitter SE through the shunt resistor, and a current flowingthrough the sense emitter SE is taken using the operational amplifierAMP10. Therefore, there is a problem that, when a current flowingthrough the sense emitter SE increases, the operational amplifier AMP10generates heat, and the current detection circuit 112 that includes theoperational amplifier AMP10 also generates heat.

In view of the above, in this embodiment, the adjustment transistor Tr3is placed between the node N1 and the low-voltage power supply Vss asshown in FIGS. 1 and 2. Then, the output of the operational amplifierAMP1 is supplied to the gate of the adjustment transistor Tr3, and acurrent flowing through the adjustment transistor Tr3 is adjusted sothat the voltage of the Kelvin emitter KE and the voltage of the senseemitter SE are substantially the same. Therefore, even when a currentflowing through the sense transistor Tr2 becomes large, it is possibleto prevent heating of the operational amplifier AMP1. Specifically, whena current flowing through the sense transistor Tr2 becomes large, acurrent flowing through the adjustment transistor Tr3 also becomeslarge; however, because this does not affect the operational amplifierAMP1, it is thereby possible to prevent the operational amplifier AMP1from generating heat.

According to the embodiment described above, it is possible to provide asemiconductor device that can suppress the heating of the currentdetection circuit even when a large current flows through the powertransistor.

<Second Embodiment>

A second embodiment is described hereinafter. FIG. 4 is a circuitdiagram showing one example of a semiconductor device according to thesecond embodiment. In a semiconductor device 2 shown in FIG. 4, anexample in which the current detection circuit 12 described in the firstembodiment is incorporated into a drive circuit (IC) 21 is shown.

As shown in FIG. 4, the semiconductor device 2 according to thisembodiment includes a power transistor circuit 11, a drive circuit 21, ashunt resistor Rs, and an adjustment transistor Tr3. Note that theconfiguration and operation of the power transistor circuit 11, theshunt resistor Rs and the adjustment transistor Tr3 are the same asthose described in the first embodiment and not redundantly described.

The drive circuit 21 drives the power transistor circuit 11. The drivecircuit 21 includes operational amplifiers AMP1 and AMP2, a comparatorCMP1, and a gate driver 22. Note that the configuration and operation ofthe operational amplifier AMP1 are the same as those of the operationalamplifier AMP1 described in the above embodiment and not redundantlydescribed.

The gate driver 22 includes a control circuit 23 and transistors Tr4 toTr6. The control circuit 23 receives a control command signal CTR andcontrols the transistors Tr4 and Tr5 according to the control commandsignal CTR. Further, the control circuit 23 controls the transistor Tr6according to the output of the comparator CMP1. The transistor Tr4 is aP-type transistor. The source of the transistor Tr4 is connected to ahigh voltage power supply Vcc, the drain of the transistor Tr4 isconnected to an output terminal OUTH, and a control signal from thecontrol circuit 23 is supplied to the gate of the transistor Tr4. Theoutput terminal OUTH is connected to the gates of the power transistorTr1 and the sense transistor Tr2 through a resistor R11.

The transistor Tr5 is an N-type transistor. The source of the transistorTr5 is connected to the ground potential, the drain of the transistorTr5 is connected to an output terminal OUTL, and a control signal fromthe control circuit 23 is supplied to the gate of the transistor Tr5.The output terminal OUTL is connected to the gates of the powertransistor Tr1 and the sense transistor Tr2 through a resistor R12. Thetransistor Tr6 is an N-type transistor. The source of the transistor Tr6is connected to the ground potential, the drain of the transistor Tr6 isconnected to an output terminal SOFT, and a control signal from thecontrol circuit 23 is supplied to the gate of the transistor Tr6. Theoutput terminal SOFT is connected to the gates of the power transistorTr1 and the sense transistor Tr2 through a resistor R13.

For example, the same level of signal is supplied to the gate of thetransistor Tr4 and the gate of the transistor Tr5. When a low levelsignal is supplied to each of the gates of the transistors Tr4 and Tr5,the transistor Tr4 becomes ON, and the transistor Tr5 becomes OFF. Atthis time, the output terminals OUTH and OUTL are HIGH, and the gates ofthe power transistor Tr1 and the sense transistor Tr2 are HIGH.Accordingly, the power transistor Tr1 and the sense transistor Tr2become ON, and a current flows through the power transistor Tr1 and thesense transistor Tr2.

On the other hand, when a high level signal is supplied to each of thegates of the transistors Tr4 and Tr5, the transistor Tr4 becomes OFF,and the transistor Tr5 becomes ON. At this time, the output terminalsOUTH and OUTL are LOW, and the gates of the power transistor Tr1 and thesense transistor Tr2 are LOW. Accordingly, the power transistor Tr1 andthe sense transistor Tr2 become OFF, and a current does not flow throughthe power transistor Tr1 and the sense transistor Tr2.

For example, by supplying a pulse control signal from the controlcircuit 23 to the gates of the transistors Tr4 and Tr5 and adjusting theduty ratio of this pulse, it is possible to adjust the amount of currentsupplied from the power transistor Tr1 to the load.

The amplifier AMP2 receives a voltage of the node N1, amplifies thevoltage of the node N1 and outputs it as a feedback signal FB. Note thatthe voltage of the node N1 corresponds to the current I_(CE) flowingthrough the power transistor Tr1, and it is possible to monitor thecurrent I_(CE) flowing through the power transistor Tr1 by monitoringthe voltage of the node N1.

A reference voltage Vref is supplied to the non-inverting input terminalof the comparator CMP1, and the voltage of the node N1 is supplied tothe inverting input terminal of the comparator CMP1. The comparator CMP1compares the reference voltage Vref and the voltage of the node N1 andoutputs a comparison result to the control circuit 23 of the gate driver22. When the comparison result from the comparator CMP1 indicates thatan overcurrent is flowing through the power transistor Tr1, the controlcircuit 23 switches the power transistor Tr1 to the OFF state.

Specifically, because a voltage of the node N1 decreases as a currentflowing through the emitter E of the power transistor Tr1 becomeslarger, when the voltage of the node N1 falls below the referencevoltage Vref, the comparator CMP1 detects an overcurrent and outputs ahigh level detection signal to the control circuit 23. When the controlcircuit 23 receives the high level detection signal from the comparatorCMP1, it outputs a high level signal to the transistor Tr6. Thetransistor Tr6 thereby becomes ON, the gate of the power transistor Tr1becomes LOW level, and the power transistor Tr1 becomes OFF.

In the comparative example shown in FIG. 7, because the voltage of thesense emitter SE is different from the voltage of the Kelvin emitter KE,bias dependence occurs in a current flowing through the sense emitterSE, which degrades the detection accuracy of the current detectioncircuit. Therefore, it is necessary to set a high detection thresholdvalue in order to prevent the lower limit of the variation of thedetection threshold from overlapping the range of a normal operatingcurrent. However, because a large current flows at the time ofshort-circuit in the power transistor such as IGBT, it is required toallow for a certain margin to the upper limit of the variation of thedetection threshold. It is thereby necessary to use a large powertransistor, which increases the cost.

On the other hand, in the semiconductor device according to thisembodiment shown in FIG. 4, because the detection accuracy of thecurrent detection circuit can be improved (see the first embodiment), itis possible to allow a low short-circuit tolerance of the powertransistor, thereby enabling use of a smaller power transistor. Forexample, because the current density of the power transistor canincrease, it is possible to obtain a necessary current with a smallerpower transistor than before.

<Third Embodiment>

A third embodiment is described hereinafter. FIG. 5 is a circuit diagramshowing one example of a semiconductor device according to the thirdembodiment. In this embodiment, a semiconductor device that has afunction of estimating the junction temperature of the power transistorcircuit 11 (particularly, the power transistor Tr1) is described.

As shown in FIG. 5, a semiconductor device 3 according to thisembodiment includes a power transistor circuit 11, a drive circuit 31, ashunt resistor Rs, and an adjustment transistor Tr3. Note that theconfiguration and operation of the power transistor circuit 11, theshunt resistor Rs and the adjustment transistor Tr3 are the same asthose described in the first embodiment and not redundantly describedbelow.

The drive circuit 31 includes a switching element SW1. The switchingelement SW1 is configured to be capable of switching between the casewhere the output of the operational amplifier AMP1 is supplied to thegate of the adjustment transistor Tr3 and the case where a voltage Vssfor turning the adjustment transistor Tr3 OFF (which is the same voltageas the source voltage of the adjustment transistor Tr3) is supplied tothe gate of the adjustment transistor Tr3. A resistor R25 and acapacitor C1 are connected in series between the output terminal of theoperational amplifier AMP1 and the ground potential.

When the switching element SW1 is connected to the output terminal ofthe operational amplifier AMP1, the operational amplifier AMP1 adjusts acurrent flowing through the adjustment transistor Tr3 so that thevoltage of the Kelvin emitter KE and the voltage of the sense emitter SEare substantially the same as described in the first embodiment. On theother hand, when the switching element SW1 is connected to thelow-voltage power supply Vss, the adjustment transistor Tr3 is forced tobe OFF. In this case, the operational amplifier AMP1 does not performfeedback control for making the voltage of the Kelvin emitter KE and thevoltage of the sense emitter SE substantially the same. Accordingly, thevoltage of the sense emitter SE becomes a voltage dependent on thejunction temperature of the power transistor circuit 11. Thus, thevoltage of the sense emitter SE in this case can be used as temperatureinformation of the power transistor Tr1.

For example, when the adjustment transistor Tr3 is turned OFF under thecondition that V_(CE) is sufficiently higher than V_(GE) and V_(GE) ishigher than V_(TH), the voltage V_(SE) of the sense emitter SE isV_(GE)−V_(TH). The voltage V_(SE) of the sense emitter SE in the ONstate where V_(CE) is lower than V_(GE) is the value of V_(SE) that isobtained by simultaneous equations of I_(SE)=f(V_(G)−V_(SE)−V_(TH)(T))and V_(SE)=R_(S)·I_(SE). V_(CE) is the collector-emitter voltage of thepower transistor Tr1, V_(GE) is the gate-emitter voltage of the powertransistor Tr1, V_(TH) is a threshold voltage of the power transistorTr1, which is a function of temperature (T), I_(SE) is a current flowingthrough the sense emitter, f(V) is a function with a voltage V as aparameter, and R_(S) is a resistance value of the shunt resistor. Thethreshold voltage V_(TH) depends on the junction temperature T_(J) ofthe power transistor Tr1 (the threshold voltage V_(TH) decreases as thejunction temperature T_(J) increases). Accordingly, the thresholdvoltage V_(TH) is estimated by comparing the voltage of the senseemitter SE and the gate voltage VG at this time, and the junctiontemperature T_(J) can be estimated from the estimation value of thethreshold voltage V_(TH).

For example, an operational amplifier AMP3 (second operationalamplifier) is placed, where the gate voltage VG of the sense transistorTr2 (which is the same voltage as the gate voltage of the powertransistor Tr1) is supplied to one input of the operational amplifierAMP3, and the emitter voltage SE of the sense transistor Tr2 is suppliedto the other input. Specifically, the sense emitter SE is connected tothe inverting input terminal of the operational amplifier AMP3 through aresistor R21, the gate of the sense transistor Tr2 is connected to thenon-inverting input terminal of the operational amplifier AMP3 through aresistor R22, and a resistor R23 is placed between the non-invertinginput terminal of the operational amplifier AMP3 and the groundpotential. Further, the output terminal and the inverting input terminalof the operational amplifier AMP3 are connected using a resistor R24. Inthis configuration, the operational amplifier AMP3 can output a voltagecorresponding to a difference between the gate voltage VG of the sensetransistor Tr and the emitter voltage SE of the sense transistor Tr2 astemperature information of the power transistor Tr1.

Further, in the case where a resistor R_(SE) is placed between the senseemitter SE and the ground potential (the resistor R_(SE) can beomitted), a measurement result depending on the junction temperatureT_(J) of the power transistor Tr1 is obtained. FIG. 8 is an example ofactual measurement the temperature dependence of the relationshipbetween a current flowing through the emitter and a current flowingthrough the sense emitter in the state where the resistor R_(SE) isplaced between the sense emitter SE and the ground potential, V_(CE) islower than V_(GE), and the adjustment transistor Tr3 is OFF. In thiscase, the junction temperature T_(J) can be estimated by comparing ameasurement result dependent on the junction temperature T_(J) (which isa current value of the sense emitter SE when the adjustment transistorTr3 is OFF) and a measurement result not dependent on the junctiontemperature T_(J) (which is a current value of the sense emitter SE whenthe output terminal of the operational amplifier AMP1 is connected tothe gate of the adjustment transistor Tr3).

A current flowing through the sense emitter SE corresponds to a voltageof the node N1. Accordingly, the temperature of the power transistor Tr1can be estimated using a voltage of the node N1 in the state where theoutput terminal of the operational amplifier AMP1 is connected to thegate of the adjustment transistor Tr3 and a voltage of the node N1 inthe case where the adjustment transistor Tr3 is OFF.

For example, an operational amplifier AMP4 is placed, where theinverting input terminal of the operational amplifier AMP4 and the nodeN1 are connected through a resistor R26, and the non-inverting inputterminal of the operational amplifier AMP4 is connected to the groundpotential. Further, the output terminal and the inverting input terminalof the operational amplifier AMP4 are connected through a resistor R27.In this configuration, the operational amplifier AMP4 outputs a voltagecorresponding to the voltage of the node N1.

In the case where the gate of the adjustment transistor Tr3 is connectedto the output terminal of the operational amplifier AMP1, theoperational amplifier AMP1 performs feedback control so that the voltageof the sense emitter SE equals the voltage of the Kelvin emitter KE.Thus, in this case, the operational amplifier AMP4 outputs a voltagethat does not depend on the junction temperature T_(J) of the powertransistor Tr1. On the other hand, when the voltage Vss is supplied tothe gate of the adjustment transistor Tr3, because the adjustmenttransistor Tr3 is OFF, the operational amplifier AMP4 outputs a voltagethat depends on the junction temperature T_(J) of the power transistorTr1. In this case, the voltage of the sense emitter SE is a voltage thatis lower than the gate-emitter voltage V_(GE) by the threshold voltageV_(TH) of the power transistor Tr1 under the condition that V_(CE) issufficiently higher than V_(GE) and V_(GE) is higher than V_(TH), and itis a more complicated function of temperature under the condition thatV_(CE) is lower than V_(GE). Accordingly, the output voltage of theoperational amplifier AMP4 is also a value corresponding to the valuethat is lower by the threshold voltage V_(TH).

FIG. 6 is a diagram illustrating the operating state of thesemiconductor device 3 according to this embodiment. As shown in FIG. 6,when the gate of the adjustment transistor Tr3 is connected to theoutput terminal of the operational amplifier AMP1, current measurement(measurement of a sense emitter current) with no temperature dependencecan be performed, and therefore this current information can be used asa measurement result of the emitter voltage I_(CE) of the powertransistor Tr1.

On the other hand, when the voltage Vss is supplied to the gate of theadjustment transistor Tr3, the adjustment transistor Tr3 is OFF, andtherefore current measurement (measurement of a sense emitter current)with temperature dependence is performed. In this case, the temperatureinformation of the junction can be obtained by performing calculation(division) using a current measurement result with no temperaturedependence and a current measurement result with temperature dependencein a computing circuit.

Further, as shown in FIG. 5, the drive circuit 31 includes signalconversion circuits 32_1 and 32_2, isolators 33_1 to 33_3, a temperatureabnormality detection circuit 34, a current abnormality detectioncircuit 35, and a gate driver 36.

The output voltage of the operational amplifier AMP3 is supplied to thetemperature abnormality detection circuit 34. The output voltage of theoperational amplifier AMP3 is a voltage corresponding to a differencebetween the gate voltage VG of the sense transistor Tr2 and the emittervoltage SE of the sense transistor Tr, and this voltage corresponds tothe temperature information of the power transistor Tr1. When the outputvoltage of the operational amplifier AMP3 indicates abnormality (forexample, when the output voltage of the operational amplifier AMP3 ishigher than a specified value), the temperature abnormality detectioncircuit 34 detects temperature abnormality of the power transistor Tr1and notifies temperature abnormality to the gate driver 36. Whentemperature abnormality is notified from the temperature abnormalitydetection circuit 34, the gate driver 36 sets the gate of the powertransistor Tr1 to LOW level to turn the power transistor Tr1 OFF.

The output voltage (temperature information) of the operationalamplifier AMP3 is supplied to MCU (Micro Control Unit) through thesignal conversion circuit 32_1 and the isolator 33_1. In the case wherethe drive circuit 31 and the MCU operate in different power supplydomains (power supply systems with different reference potentials suchas GND), it is preferred to exchange signals between them through theisolator 33_1. Because the isolator is used for transmission of adigital signal, a voltage value that is detected as an analog signal isconverted into a digital signal by the signal conversion circuit 32_1and then output to the MCU through the isolator 33_1. For the conversioninto a digital signal, a ΔΣmodulator, a pulse width modulation (PWM)circuit, an A/D converter or the like can be used, for example. Theisolator 33_1 exchanges signals in the electrically insulated stateusing optical coupling by a photocoupler or the like, magnetic couplingby a coil, a magnetoresistive element or the like, electrostaticcoupling by a parallel plate capacitor or the like and the like.

The output voltage of the operational amplifier AMP4 is supplied to thecurrent abnormality detection circuit 35. The output voltage of theoperational amplifier AMP4 corresponds to a voltage of the node N1, andthe voltage of the node N1 corresponds to current information flowingthrough the power transistor Tr1. When the output voltage of theoperational amplifier AMP4 indicates abnormality (for example, when theoutput voltage of the operational amplifier AMP4 is lower than aspecified value), the current abnormality detection circuit 35 detectsthat an overcurrent is flowing to the power transistor Tr1 and notifiescurrent abnormality to the gate driver 36. When current abnormality isnotified from the current abnormality detection circuit 35, the gatedriver 36 sets the gate of the power transistor Tr1 to LOW level to turnthe power transistor Tr1 OFF.

Further, the current abnormality detection circuit 35 can operate alsoas a temperature abnormality detection circuit. Specifically, thecurrent abnormality detection circuit 35 estimates the temperature ofthe power transistor Tr1 by using the output voltage of the operationalamplifier AMP4 in the state where the output terminal of the operationalamplifier AMP1 is connected to the gate of the adjustment transistor Tr3and the output voltage of the operational amplifier AMP4 in the casewhere the adjustment transistor Tr3 is OFF. Then, when the estimatedtemperature value is larger than a specified value, it detects that thetemperature of the power transistor Tr1 is abnormal and notifiestemperature abnormality to the gate driver 36.

The output voltage (current information) of the operational amplifierAMP4 is supplied to MCU (Micro Control Unit) through the signalconversion circuit 32_2 and the isolator 33_2. Further, a signal fromthe MCU is supplied to the gate driver 36 through the isolator 33_3.Note that the configuration of the signal conversion circuit 32_2 is thesame as that of the signal conversion circuit 32_1, and theconfiguration of the isolators 33_2 and 33_3 is the same as that of theisolator 33_1.

As described above, in the semiconductor device 3 according to thisembodiment, by switching between the case where the output of theoperational amplifier AMP1 is supplied to the gate of the adjustmenttransistor Tr3 and the case where the voltage Vss for turning theadjustment transistor Tr3 OFF is supplied to the gate of the adjustmenttransistor Tr3, it is possible to estimate the junction temperature ofthe power transistor Tr1. This eliminates the need to include a diodefor temperature detection in the power transistor circuit 11.

Further, because a diode for temperature detection is often formed in apolysilicon layer which is different from a diffusion layer of the powertransistor circuit 11, there are large variations in characteristics andthe measured temperature is different from the temperature of thediffusion layer in some cases. On the other hand, in the semiconductordevice 3 according to this embodiment, because the junction temperatureis estimated using the temperature characteristics of the powertransistor Tr1 itself, it is possible to accurately measure thetemperature of the power transistor Tr1.

The first to third embodiments can be combined as desirable by one ofordinary skill in the art.

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention can bepracticed with various modifications within the spirit and scope of theappended claims and the invention is not limited to the examplesdescribed above.

Further, the scope of the claims is not limited by the embodimentsdescribed above.

Furthermore, it is noted that, Applicant's intent is to encompassequivalents of all claim elements, even if amended later duringprosecution.

What is claimed is:
 1. A semiconductor device comprising: a powertransistor; a sense transistor having a gate connected to a gate of thepower transistor and a collector or drain connected to a collector ordrain of the power transistor; a first operational amplifier having afirst input terminal connected to an emitter or source of the sensetransistor and a second input terminal connected to an emitter or sourceof the power transistor; a resistor element having one end connected tothe emitter or source of the sense transistor and another end connectedto a first node; and an adjustment transistor placed between the firstnode and a low-voltage power supply and having a base or gate suppliedwith an output of the first operational amplifier, wherein the firstoperational amplifier adjusts a current flowing through the adjustmenttransistor so that a voltage of the emitter or source of the powertransistor and a voltage of the emitter or source of the sensetransistor are substantially the same, and wherein the adjustmenttransistor is placed in a first semiconductor chip and the firstoperational amplifier is placed in a second semiconductor chip which isdifferent from the first semiconductor chip.
 2. The semiconductor deviceaccording to claim 1, wherein the first input terminal of the firstoperational amplifier is a non-inverting input terminal, and the secondinput terminal of the first operational amplifier is an inverting inputterminal, and a collector or drain of the adjustment transistor isconnected to the first node, and an emitter or a source of theadjustment transistor is connected to the low-voltage power supply. 3.The semiconductor device according to claim 1, wherein a current flowingthrough the power transistor is monitored by monitoring a voltage of thefirst node.
 4. The semiconductor device according to claim 2, whereinthe inverting input terminal of the first operational amplifier isconnected to a ground potential, and emitter of the source of theadjustment transistor is connected to a negative potential.
 5. Thesemiconductor device according to claim 1, further comprising: a gatedriver that drives the power transistor; and a comparator that comparesa voltage of the first node with a specified reference voltage andoutputs a comparison result to the gate driver, wherein the gate driverturns the power transistor OFF when the comparison result from thecomparator indicates that an overcurrent is flowing through the powertransistor.
 6. The semiconductor device according to claim 1, furthercomprising: a switching element capable of switching between a casewhere the output of the first operational amplifier is supplied to thegate of the adjustment transistor and a case where a voltage for turningthe adjustment transistor OFF is supplied to the gate of the adjustmenttransistor, wherein a temperature of the power transistor is estimatedby using a voltage of the first node measured when the output of thefirst operational amplifier is supplied to the gate of the adjustmenttransistor and a voltage of the first node measured when the adjustmenttransistor is OFF.
 7. A semiconductor device comprising: a powertransistor; a sense transistor having a gate connected to a gate of thepower transistor and a collector or drain connected to a collector ordrain of the power transistor; a first operational amplifier having afirst input terminal connected to an emitter or source of the sensetransistor and a second input terminal connected to an emitter or sourceof the power transistor; a resistor element having one end connected tothe emitter or source of the sense transistor and another end connectedto a first node; and an adjustment transistor placed between the firstnode and a low-voltage power supply and having a base or gate suppliedwith an output of the first operational amplifier, wherein the firstoperational amplifier adjusts a current flowing through the adjustmenttransistor so that a voltage of the emitter or source of the powertransistor and a voltage of the emitter or source of the sensetransistor are substantially the same, and wherein the low-voltage powersupply of the first operational amplifier and the low-voltage powersupply connected to the adjustment transistor are lower than a voltageof the emitter or source of the power transistor.
 8. A semiconductordevice comprising: a power transistor; a sense transistor having a gateconnected to a gate of the power transistor and a collector or drainconnected to a collector or drain of the power transistor; a firstoperational amplifier having a first input terminal connected to anemitter or source of the sense transistor and a second input terminalconnected to an emitter or source of the power transistor; a resistorelement having one end connected to the emitter or source of the sensetransistor and another end connected to a first node; an adjustmenttransistor placed between the first node and a low-voltage power supplyand having a base or gate supplied with an output of the firstoperational amplifier, wherein the first operational amplifier adjusts acurrent flowing through the adjustment transistor so that a voltage ofthe emitter or source of the power transistor and a voltage of theemitter or source of the sense transistor are substantially the same;and a switching element capable of switching between case where theoutput of the first operational amplifier is supplied to the gate of theadjustment transistor and a case where a voltage for turning theadjustment transistor OFF is supplied to the gate of the adjustmenttransistor, wherein a voltage of the emitter or source of the sensetransistor when the adjustment transistor is OFF is used as atemperature information of the power transistor.
 9. The semiconductordevice according to claim 8, further comprising: a second operationalamplifier having one input supplied with a gate voltage of the sensetransistor and another input supplied with a voltage of the emitter orsource of the sense transistor, wherein the second operational amplifieroutputs a voltage corresponding to a difference between the gate voltageof the sense transistor and the voltage of the emitter or source of thesense transistor as the temperature information of the power transistor.10. The semiconductor device according to claim 8, further comprising: atemperature abnormality detection circuit that detects temperatureabnormality of the power transistor by using the temperature informationof the power transistor; and a gate driver that drives the powertransistor, wherein the gate driver turns the power transistor OFF whenthe temperature abnormality detection circuit detects temperatureabnormality of the power transistor.